checkout.c: introduce an *_INIT macro

Add an *_INIT macro for the tracking_name_data similar to what exists
elsewhere in the codebase, e.g. OID_ARRAY_INIT in sha1-array.h. This
will make it more idiomatic in later changes to add more fields to the
struct & its initialization macro.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

checkout.c

@@ -10,6 +10,8 @@ struct tracking_name_data {
 	int unique;
 };
 
+#define TRACKING_NAME_DATA_INIT { NULL, NULL, NULL, 1 }
+
 static int check_tracking_name(struct remote *remote, void *cb_data)
 {
 	struct tracking_name_data *cb = cb_data;
@@ -32,7 +34,7 @@ static int check_tracking_name(struct remote *remote, void *cb_data)
 
 const char *unique_tracking_name(const char *name, struct object_id *oid)
 {
-	struct tracking_name_data cb_data = { NULL, NULL, NULL, 1 };
+	struct tracking_name_data cb_data = TRACKING_NAME_DATA_INIT;
 	cb_data.src_ref = xstrfmt("refs/heads/%s", name);
 	cb_data.dst_oid = oid;
 	for_each_remote(check_tracking_name, &cb_data);